Methane and other light hydrocarbon combustibles are often found in remote areas. Worldwide natural gas reserves have increased on an average of about six percent annually, while natural gas consumption has only increased about three percent annually.
The difference between known and used reserves has increased, therefore, to about 4.6 quadrillion cubic feet. Known natural gas reserves, therefore, have an energy equivalent of some 770 billion barrels of oil that is equivalent to about a 29 year worldwide supply of oil for energy purposes. Unfortunately, however, most of the natural gas reserves are located in remote areas. Remote natural gas reserves usually make the economics of extraction and removal unfeasible.
The Fischer-Tropsch process, developed early in the 20th century in Germany, uses fossil fuels and converts the fossil fuels to liquid synthetic gasoline species. The Fischer-Tropsch synthesis is strongly exothermic and often requires hydrogen in the process. Where a Fischer-Tropsch process is being conducted at remote sites without a proper infrastructure for readily available hydrogen, the cost of production is significantly increased by the need to bring the hydrogen to the remote site.
As environmental concerns increase regarding greenhouse gases that may contribute to global warming, increased interest is directed toward finding clean burning fuels that do not produce carbon dioxide emissions. Hydrogen as a fuel seems to be ideal as it burns to form only water as its combustion product.
A need has long existed for converting available carbonaceous materials to scarce liquid hydrocarbon fuels having preferred performance characteristics in many applications, such as internal combustion engines, jet engines and open-cycle gas turbines. Thus, for example, U.S. Pat. No. 3,986,349 teaches a process for converting solid coal to a liquid hydrocarbon fuel by gasifying the coal to a synthesis gas, hydrogenating the resulting synthesis gas, and recovering a liquid hydrocarbon fuel from the hydrogenation product. The liquid hydrocarbon fuel is used to generate power by relatively clean combustion in an open-cycle gas turbine.
An alternative is to produce natural gas and convert it in the field to a more utilitarian liquid hydrocarbon fuel or liquid chemical product for local usage or for more cost-effective transportation to remote markets. Processes for converting light hydrocarbon gases, such as natural gas, to heavier hydrocarbon liquids are generally known in the prior art. Such processes typically involve the indirect conversion of methane to synthetic paraffinic hydrocarbon compounds, wherein methane is first converted to a synthesis gas containing hydrogen and carbon monoxide followed by conversion of the synthesis gas to synthetic paraffinic hydrocarbon compounds via a Fischer-Tropsch reaction. The unconverted synthesis gas remaining after the Fischer-Tropsch reaction is usually catalytically reconverted to methane via a methanation reaction and recycled to the process inlet to increase the overall conversion efficiency of the process.
Conversion of methane to a synthesis gas is often performed by high-temperature steam reforming, wherein methane and steam are reacted endothermically over a catalyst contained within a plurality of externally-heated tubes mounted in a large fired furnace. Alternatively, methane is converted to a synthesis gas via partial-oxidation, wherein the methane is exothermically reacted with purified oxygen. Partial oxidation using purified oxygen requires an oxygen separation plant having substantial compression capacity and correspondingly having substantial power requirements. Production of the synthesis gas via either of the above-recited methods accounts for a major portion of the total capital cost of a plant for converting methane to paraffinic hydrocarbons.
Autothermal reforming is a lower cost method of converting methane to a synthesis gas. Autothermal reforming employs a combination of partial oxidation and steam reforming. The endothermic heat required for the steam reforming reaction is obtained from the exothermic partial oxidation reaction. Unlike the above-recited partial oxidation reaction, however, air is used as the source of oxygen for the partial oxidation reaction. In addition, the synthesis gas produced by autothermal reforming contains substantial quantities of nitrogen from the inlet air. Consequently, it is not possible to recycle the unconverted components contained in the process tail gas without undesirably accumulating an excess of nitrogen within the process. Production of a nitrogen-diluted synthesis gas via autothermal reforming or partial-oxidation using air followed by conversion of the synthesis gas via a Fischer-Tropsch reaction as disclosed in U.S. Pat. Nos. 2,552,308 and 2,686,195 is, nevertheless, a useful method for obtaining synthetic hydrocarbon liquid products from methane.
U.S. Pat. No. 4,833,170 discloses another example of autothermal reforming, wherein a gaseous light hydrocarbon is reacted with air in the presence of recycled carbon dioxide and steam to produce a synthesis gas. The synthesis gas is reacted in the presence of a hydrocarbon synthesis catalyst containing cobalt to form a residue gas stream and a liquid stream comprising heavier hydrocarbons and water. The heavier hydrocarbons are separated from the water and recovered as product. The residue gas is catalytically combusted with additional air to form carbon dioxide and nitrogen which are separated. At least a portion of the carbon dioxide is recycled to the autothermal reforming step.
Although prior art hydrocarbon gas conversion processes such as disclosed in U.S. Pat. No. 4,833,170 may be relatively effective for converting the light hydrocarbon gases to heavier hydrocarbon liquids, such processes have not been found to be entirely cost effective due to significant capital equipment and energy costs attributable to compression of the inlet air. The power required to compress the inlet air represents the majority of the mechanical power required to operate the process, yet much of this power is essentially lost as unrecovered pressure energy in the residue gas from the process. The inlet air requiring compression contains substantial quantities of nitrogen that remain essentially chemically inert as the nitrogen passes through the process, ultimately exiting the process in the residue gas. Furthermore, although the residue gas has a significant chemical-energy fuel value attributable to the carbon monoxide, hydrogen, methane and heavier hydrocarbon components thereof, the residue gas is very dilute, having a low heating value that renders it very difficult and costly to recover the energy of the fuel value of the residue gas with high efficiency. Thus, it is apparent that a need exists for a more cost-effective hydrocarbon gas conversion process.
In the above-mentioned technologies, it is well known that a carbon-containing chemical species will be combusted and the amount of greenhouse gases that are emitted to the atmosphere, namely carbon dioxide, is increased. What is needed in the art is a process for making synthetic fuels from light hydrocarbons that eliminates the emission of greenhouse gases.
Another problem that occurs is the need for cryogenic storage of converted fuels such as hydrogen. Hydrogen storage presents a problem because an on-board system of a hydrogen heat engine for a vehicle has a range of approximately 50 miles with hydrogen stored in pressurized tanks. Typically, a combination of both cryogenic and high-pressure hydrogen storage are required in order to contain the hydrogen in a compact enough package to carry as an on-board system.
Several attempts have been made to store hydrogen as a metal hydride in order to lessen the need for both cryogenic and high-pressure storage. Metal hydride storage has its own challenges including added weight of the metal and added energy required to separate the hydrogen as the hydride of the metal in order to provide it as the fuel source. What is needed in the art is a process of making synthetic fuels from light hydrocarbons that eliminates the problems of hydrogen storage experienced in the prior art.
Another problem that exists in the prior art is the creation of H2 and soot by use of a plasmatron. Although H2 and soot may be created, such as taught by Bromberg et al. in U.S. Pat. No. 5,409,784, non-ultrafine soot particle sizes are irregular and tend to form agglomerations that are of a size above the 2,000 nm range. Although plasmatrons may produce H2 and soot, the extremely chaotic nature of the production of H2 and soot likely cause the irregular soot particle sizes.